\documentclass{article}
\pagestyle{empty}

\usepackage{tikz}
\usetikzlibrary{shapes,shapes.geometric,arrows}
\begin{document}

\input{tikz_styles.tex}

\tikzstyle{input} = [coordinate]
\tikzstyle{output} = [coordinate]

\begin{tikzpicture}[auto, thick, node distance=1.5cm, >=stealth']

  \node [input, name=input] {};
  \node [sum, right of=input] (sum) {};
  \node [block, right of=sum] (F) {$F(s)$};
  \node [block, right of=F] (N) {$N(s)$};
  \node [coordinate, right of=N, node distance=1cm] (branch) {};
  \node [output, name=output, right of=branch] {};
  \node [coordinate, below of=branch, node distance=1cm] (foo) {};

  \draw [->] (input) edge node {$\theta_{\rm in}$} (sum);
  \draw [->] (sum) -- (F);
  \draw [->] (F) -- (N);
  \draw [->] (N) edge node {$\theta_{\rm out}$} (output);
  \draw [-] (branch) -- (foo);
  \draw [->] (foo) -| (sum);

  %\node [above of=input, node distance=0.3cm] {$\theta_{\rm in}$};
  %\node [above of=output, node distance=0.3cm] {$\theta_{\rm out}$};

  \draw [-,thin] (sum.north west) -- (sum.south east);
  \draw [-,thin] (sum.north east) -- (sum.south west);

\end{tikzpicture}

\end{document}

